Electrodes produced by the following methods have been proposed in the past as electrodes for electrochemical apparatuses in which a solid polymer electrolyte is used.
(1) A layer composed of a mixture of polytetrafluoroethylene (PTFE) or another such water-repellent binder and carbon black is formed on the surface or in the internal voids of carbon paper or the like that has undergone a water repellency treatment with PTFE or the like, and a catalyst layer is formed on the surface of this first layer, after which this product is joined with a solid polymer electrolyte membrane by hot pressing or the like. PA1 (2) A paste-like or ink-like liquid, obtained by mixing a PTFE dispersion with a catalyst substance, or by mixing a solid polymer electrolyte resin solution, or a resin solution of a precursor substance thereof, with a catalyst substance, is directly applied to a solid polymer electrolyte membrane, and this coating is dried, heated, and molded, or is further molded by hot pressing. PA1 (3) A paste-like or ink-like liquid, obtained by mixing a PTFE dispersion with a catalyst substance, or by mixing a solid polymer electrolyte resin solution, or a resin solution of a precursor substance thereof, with a catalyst substance, is applied over a release sheet, such as a PTFE sheet, and this coating is dried and heated, and the catalyst layer thus formed is joined by hot pressing or the like to a solid polymer electrolyte membrane, after which the release sheet is peeled off.
The present invention relates to a gas diffusion layer/collector that is used for a solid polymer electrolyte membrane electrode assembly formed by Method 2 or 3 above, or by a similar method.
Materials known in the past for use in a gas diffusion layer/collector such as this comprised carbon paper, or carbon paper that had undergone a water repellency treatment, or a layer composed of carbon black mixed with a fluororesin (used as a binder/water repellant) and formed on the surface of this carbon paper, or this mixture packed into the pores of carbon paper.
The use of carbon fiber woven cloth in place of this carbon paper has also been suggested (eg, U.S. Pat. No. 4,647,359 and Japanese Laid-Open Patent Application 58-165254).
However, a drawback to the known gas diffusion layers/collectors discussed above was that, although the conductivity in the surface direction was good since carbon paper has a structure in which carbon fibers are linked together with carbon, the conductivity in the thickness direction was lower than that in the surface direction. As to the mechanical properties, the rigidity was high, but the material was relatively brittle and had poor elasticity, so when more than a certain amount of pressure was applied in an attempt to ensure good electrical contact, the structure readily broke and the conductivity and air permeability decreased. Furthermore, although the structure of carbon paper did result in good air permeability in the thickness direction, the air permeability in the surface direction was not very good, so not much gas diffusion in the surface direction could be expected. Consequently, gas diffusion was blocked by the rib protrusions that formed the gas channel in the fuel cell separator, and as a result, there was a decrease in cell performance.
A carbon fiber woven cloth, on the other hand, is pliant, with none of the mechanical brittleness mentioned above, and depending on the fiber structure and how the material is folded, it is possible to impart resilience in the thickness direction. However, a carbon fiber woven cloth has unstable electrical contact, that is, resistance, because the fibers are not fixed, and there is also the danger that excessive pliancy will result in a loss of the form of contact with the electrolyte or catalyst electrode. In view of this, it has been proposed that a mixture composed of a fluororesin and carbon black be completely packed into the voids of a carbon fiber woven cloth when a conventional carbon fiber woven cloth is used, as discussed in U.S. Pat. No. 4,647,359 and Japanese Laid-Open Patent Application 58-165254. This method does indeed afford a certain amount of rigidity and stable conductivity, but because the voids are packed with a fluororesin and carbon black, which have poor gas permeability, the gas diffusion is particularly bad in the surface direction, so the advantage of using a carbon fiber woven cloth is lost.
These gas diffusion layers/collectors have been investigated in conjunction with fuel cells that make use of an electrolytic solution, and more specifically, primarily phosphoric acid types of fuel cells. Accordingly, the materials need to function not only as a gas diffusion layer, but also as an electrode, and have been proposed as materials having a catalyst in their structure. Therefore, these materials have not been studied for use in solid polymer electrolyte fuel cells that do not make use of an electrolytic solution, and so did not necessarily have an optimal structure for a gas diffusion layer/collector.
The inventors arrived at the present invention as a result of investigation into a gas diffusion layer/collector that can be mass produced easily and that can be used optimally in a fuel cell not of a conventional type in which a electrolytic solution is used as discussed above, but rather of a type in which a solid polymer electrolyte is used, and of a type in which a catalyst layer is joined ahead of time with a solid electrolyte membrane, that is, a type in which the above-mentioned membrane electrode junction (2) or (3) is used.